This invention relates to an electric power control device for loads of resistance heating elements in an automatic temperature regulator or the like.
Conventional electric power control devices employed for commercial power sources and making use of semiconductor switches generate high-frequency noises or cause fluctuations in voltage on power source lines, since they turn on or off relatively large powers.
When a heater load is ON-OFF controlled in a control operation in which the regulation of temperature is required, the maximum heat quantity is produced while the heater is kept on. For this reason, mere ON-OFF control is unable to meet the demand for fine adjustment of heat to be generated.
One possible method for achieving fine adjustment of the quantity of produced heat is to control the heater operation by firing thyristors. Although this method is effective for achieving fine adjustment in the regulation of temperature, high-frequency noises are produced resulting in deleterious effects to radio receivers and T.V. receivers. This noise is the result of the abrupt rise of current upon firing as illustrated in FIG. 3(a) which illustrates the waveform of a terminal voltage of a load and the waveform of a voltage on the corresponding power source line in the conventional electric power control method.
Alternatively, the ON-OFF control of a heater may be effected at the instant that the ac supply voltage passes through zero. This method makes it possible to perform the ON-OFF control more frequently without generating high-frequency noises.
FIG. 3(b) illustrates the waveform of a terminal voltage of a load and the waveform of a voltage on the corresponding power source line in the conventional electric power control method. As shown in FIG. 3(b) the ON-OFF control of the shortest ON time of which is set equal to a single cycle of the voltage of an ac power source causes a large current to flow in the circuit upon turning on and off the electric power, thereby producing an abrupt voltage change (.DELTA.V) on the power source line and hence leading to such a drawback that fluorescent lamps are caused to flicker.